Hydraulic tools



Filed April 26, 1965 3 Sheets-Sheet l w W lt? i551 June 20, 1967 c, JOHNSON 3,326,304

HYDRAULIC TOOLS Filed April 26, 1965 3 $1169LS-Sh68i 2 June 20, 1967 c. D. JOHNSON HYDRAULIC TOOLS Filed April 26, 1965 3 Sheets-Sheet 3 ffa,

via $1. M

United States Patent 3,326,304 HYDRAULIC TOOLS Charles D. Johnson, Fairmont, Minn., assignor to Fairmont Railway Motors, Inc., Fairmont, Minn., a corporation of Minnesota Filed Apr. 26, 1965, Ser. No. 453,553 7 Claims. (Cl. 173169) This is a continuation-in-part of application Ser. No. 379,566, filed July 1, 1964, now abandoned.

This invention pertains to power tools and more particularly to hydraulically operated tools that are normally to be supported and manipulated by an individual operator. Such tools may be defined as hand tools, and include wrenches, drills, chain saws, limb saws, pruners and the like.

Tools of the type to which this invention pertains must be provided with adequate power to readily accomplish the tasks to which they may be applied. These tools often must be manipulated and operated in difficult circumstances, as among tree limbs or brush, or in and around structures or machinery, and may be operated from precarious positions, as atop ladders, on poles, or in boom riggings. Accordingly, it is also desirable that the tools be light in weight and convenient to handle. The external design of the units should be as smooth and as streamlined as possible, with a minimum number of projections or parts which might catch on tree limbs, twigs, machinery, and the like and be broken or impede movement and use of the device. Further it is desirable that such power operated devices be of a rugged design with a minimum number of parts to provide long trouble-free service.

The use of hydraulic power systems to drive tools is of course known. Such systems permit the use of high operating pressure, whereby high power output can be obtained at the tool with a relatively small, compact prime mover. So-called open-center control valves are advantageous in such systems in that when the control valve of the tool is in the oflf position, the valve is open for circulation of the hydraulic fluid from the pressure side of the source to the return or sump side, thereby avoiding any need of an unloader system and providing cooling of the fluid as it circulates through the supply lines.

It is an object of this invention to provide improved hydraulic tools of the type indicated above and meeting the aforementioned requirements.

It is another object of this invention to provide improved hydraulic hand tools.

It is another object of this invention to provide hydraulic hand tools which are convenient and easy for the operator to manipulate and use.

It is another object of this invention to provide improved hydraulic tools of a simple, economical and rugged construction.

it is another object of this invention to provide improved hydraulic tools of the type operated by a reciprocating drive piston.

It is another object of this invention to provide a hydraulically operated pruner having a minimum number of parts and which is of rugged construction and economical to produce.

In one embodiment of this invention a pruner is provided comprising an elongated tubular member having a cutting mechanism mounted at one end and an elongated hydraulic drive and control unit mounted at the other end. The cutting mechanism includes a movable cutting member adapted to be actuated by reciprocation of an operator rod which extends longitudinally through the tubular member. The hydraulic unit comprises a cylinder secured at one end to said tubular member and extending in generally axial alignment therewith, a piston reciprocable in said cylinder, and a connecting rod secured to said piston at one end and joined to said operating member. A hand grip extends from the end of said cylinder remote from said tubular member. Pressure and return conduits for hydraulic fluid extend through said hand grip and communicate with appropriate connections for hydraulic fluid pressure and return lines at the distal or butt end of the hand grip. Said pressure conduit also communicates with the end of said cylinder receiving said connecting rod, and said return conduit communicates with the opposite end of said cylinder. A control valve is provided to control the flow of hydraulic fluid between said conduits. A relief valve is positioned in the return conduit in said hand grip to prevent return tflow of hydraulic fluid from said cylinder and conduits except when the pressure therein exceeds a predetermined minimum value at which the force differential exerted on said piston due to the cross-sectional area of the piston rod will cause return movement of said piston, and hence movement of the cutting member to its open or inoperative position. When said control valve is closed the pressure available in said pressure line is applied to the connecting rod side of said piston to effect a power or cutting stroke of said piston.

For a more complete understanding of this invention, reference should now be had to the drawings, wherein:

FIGURE 1 is a partially cross-sectional side view of a pruner employing the teachings of this invention;

FIG. 2 is an enlarged cross-sectional view of the hydraulic drive unit of the pruner in FIG. 1;

FIG. 3 is a cross-sectional view taken along line 33 of FIG. 2 and looking in the direction of the arrows;

FIG. 4 is an enlarged cross-sectional view of the valve spool of the control valve illustrated in FIG. 2;

FIG. 5 is a fragmentary side view of a chain saw, partially in cross section, illustrating another modification of a tool handle and control unit employing teachings of this invention;

FIG. 6 is a fragmentary cross-sectional view of a limb saw illustrating another modification of a tool handle and control unit employing teachings of this invention;

FIG. 7 is a side view, partially in cross section, of a power wrench or drill, illustrating another modification of a handle and control unit employing teachings of this invention;

FIG. 8 is a plan view of a part of the handle of the tool of FIG. 7;

FIG. 9 is an enlarged top plan view of the directional valve spool of the tool in FIG. 7; and

FIG. 10 is a side elevation view of the valve spool of FIG. 9.

Referring particularly to FIGS. 1 and 2, the illustrated pruner comprises generally an elongated hollow tubular member 10 having a cutting element 11 mounted on one end thereof and a hydraulic operating unit 12 mounted on the opposite end. An operator rod 13 extended through the hollow center of member 10 and interconnects the cutting unit 11 with the hydraulic operating unit 12. Operator 13 may include portions of increased diameter as at 14 and 15 to serve as locators and stabilizers for the relatively slim rod 13 in the hollow center of member 10. Member 10 may be of any suitable material such as steel, aluminum, resin-reinforced glass fibers or the like and is preferably of a relatively lightweight strong material. A suitable gasket or other sealing member may be provided as at 16 to prevent ingress of dirt, dust or other foreign material into the hollow center of member 10. A generally U-shaped bracket 17 is secured to the end of member 10 as at 18, and serves as a guide for a roller 19 on the outer end of operator 13.

The cutting element 11 includes a pair of parallel, spaced, hook-shaped members which are rigidly secured to bracket 17 and pivotally support an arcuate shear blade 21 therebetween. A pair of spaced parallel links 22 connect the outer end of operator rod 13 to the outer end of pivoted blade 21 whereby the arcuate cutting edge 21a is forcibly moved toward the hooked portions 23 of hooks 20 as the operator rod is retracted. It .will be appreciated that other cutting mechanism may be utilized in pruners embodying this invention, the specific construction described above being illustrative of one operable form.

The hydraulic drive unit 12 includes an integral cast housing 24which is formed with a number of internal bores and passageways as will be described below. A cylindrical bore 25 at one end of housing 24 receives a reciprocal piston 26. A piston rod 27 has one end connected to piston 26, and is operatively connected to the adjacent end of operator 13. Cap member 28 closes the outer end of cylinder 25, around rod 27 and appropriate seals such as O-rings at 29 and 30 are provided to insure sealing engagement and prevent leakage of hydraulic fluid from cylinder 25. A similar seal 31 is provided on piston 26.

The base or rear end of housing 24, at 32, is designed to serve as a hand grip for the operator. Grip 32 is of generally oval cross section, see FIG. 3, and is tapered from the rear end forward with a smoothly curved fillet 33 at the'forward end, see FIG. 2. A pair of passageways 34 and 35 extend longitudinally of the housing 24 from the base end of the grip portion 32, and communicate with opposite ends of the cylinder 25 as illustrated. A valve bore 36 extends transversely of housing 24 just forward of the grip portion and includes a passage 36a communicating with passageways 34 and 35.

A valve spool'37 is disposed in bore 36 and is provided with a land 38 of a size to close the passage 36a between passageways 34 and 35, and with a groove 39 of a length greater than the thickness of the housing portion 24a between the passageways 34 and 35. Closure plates 40 and 41 are provided at opposite ends of bore 36, plate 41 being provided with an aperture of a size to pass a reduced diameter portion 42 .of spool 37 but to engage a shoulder thereon at 43. A compression spring 44 is confined between plate 40 and the corresponding end of the spool 37 to bias the spool towards the position illustrated in FIG. 2 wherein communication is provided between passageways or conduits 34 and 35 through passage 36a. Appropriate seals, for instance of the O-ring type, are provided at 45 and 46 to prevent leakage of hydraulic fluid'out the ends of bore 36. Spool 37 is formed with a pressure relief passageway 47 (see FIG. 4) which communicates with groove 39 and with the annular groove --48 which receives seal 46 to prevent pressure build-up on seal 46 and attendant binding of the spool 37.

An operating or trigger member 49 is pivotally mounted on the housing 24 as at 50. Member 49 engages the protruding end 42 of spool 37 and extends adjacent and generally parallel to the grip portion 32 for convenient manipulation by the hand of an operator on the grip portion.

A relief valve 51 is provided at the outlet end of return flow passageway 34. This valve may take any appropriate form and is illustrated as comprising a ball 52 engaging a 'seat 53 under the force of a compression spring 54. The opposite end of the spring 54 engages a spider 55 extending across the opposite end of the valve element. Hydraulic line connectors 56 and 57 are connected to the outlet end of passageway 34 and the inlet end of passageway 35 as illustrated. These connectors extend rearwardly from the grip portion 32, generally parallel to the axis of the grip, whereby the adjacent end portions of suitable hydraulic lines (not shown) connected thereto will normally extend directly away from the gripped portion, generally axially of the tool.

In operation, a hydraulic pressure line from a suitable hydraulic pump (not shown) is connected with connector 57, and a return line, normally leading to the sump or reservoir of the pump, is connected to connector 56. With spool 37 in its normal open position as illustrated in FIG. 2, hydraulic pressure will be built up in conduits 34 and 35 and in cylinder 25 until the pressure overcomes relief valve 51 and return flow to the reservoir begins. The net effective areas of application of the hydraulic pressure on the opposite sides of piston 26 differ by an amount equal to the cross-sectional area of piston rod 27 where it passes through the forward end wall or cap member 28. That is, the effective pressure area of the rear face 26a of piston 26 and the associated elements for driving piston 26 forward exceeds the effective pressure area of the forward face 26b for driving piston 26 rearward by the area of cross section of connecting rod 27. Thus, assuming the pressure on both sides of piston 26 to be equal, a net force equivalent to the pressure times the cross-sectional area of rod 27 will be exerted on the piston 26, toward the left in FIG. 2. However, if the flow of fluid from passageway 34 to the pump reservoir is unrestricted, the pressure drop across restricted passageway 36a is sufficient to cause the force on side 26b to be greater than the force on side 26a, or at least to prevent the force on side 26a from overcoming the force on side 26b plus the friction of the system. Relief valve 51 restricts the return flow of the fluid and raises the back pressure in passageway 34, thereby reducing the pressure drop across passageway 36a. This relief valve is selected and adjusted to provide sufficient back pressure to insure movement of the piston 26 to the left, as seen in FIG. 2, and thus to open the cutting mechanism.

To operate the tool for cutting a limb or the like, the object to be cut is engaged in hooks 2t) and the operator depresses trigger 49. The trigger shifts spool 37 to a position where land 38 closes passage 36a. Thereupon hydraulic pressure builds up in pasasgeway 35 and in the forward end of cylinder 25, around piston rod 27, to the hydraulic source pressure, while the opposite side of the piston is vented to the return line through passageway 34 and the relief valve 51. Assuming that the source pressure sufficiently exceeds the relief pressure of valve 51, piston 26 is thus driven rearwardly, to the right in FIG. 2, and thereby drives blade 21 to its closed position to perform the cutting operation.

Bore 25 is provided with an enlarged portion at 58. Piston 26 may be provided with an annular flange or other protrusion of reduced diameter, such as the flange 59a on sleeve element 59, to cooperate with a shoulder 58a and form a fluid trap to provide a dashpot action for cushioning the termination of the power stroke of piston 26. Upon release of trigger 49, valve 37 opens and both sides of piston 26 return to substantially the same pressure, under the influence of valve 51, to drive the piston through its return stroke as described above. The piston thus is retracted automatically to open the cutter blade.

A booster spring 59b may optionally be included to receive and store energy at the end of the power stroke for subsequently assisting in opening the shear blade 21. The booster spring 5% is compressed between piston 26 and the sleeve 59, this sleeve being slidably mounted on a cap screw 59c engaged in nut 27a on the end of piston rod 27 As the piston nears the rearward end of its power or cutting stroke, flange 59a of sleeve 59 engages shoulder 58a. Thereupon, further rearward movement of the piston (to the right in FIG, 2) results in movement of the piston 26 toward flange 59a and, hence, compression of spring 59b. Subsequently, when the valve 37 opens, the spring 59b supplies a supplementary return force to the piston for a short distance to aid in opening the blade 21. This is particularly advantageous in the event the blade becomes jammed at the end of the cutting stroke, for instance, when a chip becomes wedged between blade 21 and either of the hooks 20. In one commercial embodiment, a spring 59]) was placed under a compressive pre-load between piston 26 and flange 59a, and sleeve 59 had an operative stroke of about on cap screw 590. In this unit, the spring provided a booster force of about 280 lbs. to aid in opening the blade 21.

It will be appreciated that the aforedescribed hydraulic drive is very simple in construction yet provides a positive return or opening stroke of the operating mechanism with an open-center hydraulic control valve, and is a compact system without external encumbering parts. Further, a convenient handle is provided with an adjacent operating trigger, and the connections for external hydraulic lines are so located as to be near the handle or hand grip on the tool yet out of the way of the operator whereby the tool is convenient and easy to manipulate and use.

It will be readily apparent to those skilled in this art that teachings of this invention may be modified and used in other specific embodiments. For instance thesimple control system for the reciprocating hydraulic drive may be utilized in other tools requiring a power stroke in only one direction and having a relatively light load on the return stroke. The integral assembly as illustrated represents a simple and preferred embodiment Further, the unitary and convenient handle control arrangement may be modified and incorporated in many other hydraulic tools, for instance in the modifications illustrated in FIGS. 5 through and described below. Similar parts bear similar numbers in the various illustrated modifications, but with different prefixes as will be evident.

The modification of FIG. 5 comprises the handle and power control of a chain saw shown in part at 160 and includes a housing 124 defining a grip portion 132, passageways 134 and 135, a valve bore 136, control valve spool 137, trigger member 149 and connectors 156 and 157, all similar to corresponding parts in the modification of FIG. 1. No relief valve is necessary in this tool. The hydraulic motor 161 is drive connected to the saw chain 162, and a subplate portion of housing 124 is formed with internal passageways or conduits 163 and 164 for conducting the hydraulic fluid between the passageways 134 and 135 and appropriate ports of motors 161. Cavity 165 is exposed to hydraulic fluid under pressure, through groove 139, when the valve 137 is shifted to the on or tool operating position, and cavity 165 is connected to a metering valve (not shown) for automatic lubrication of the saw chain 162. A trigger guard 166 protects trigger member 149.

The modification of FIG. 6 is quite similar to that of FIG. 5 and includes a housing 224 defining a hand grip 232, passageways 234 and 235, bore 236 with a valve 237, trigger 249, connectors 256 and 257, and a guard 266. However, this handle unit is adapted for use as part of a limb saw or similar device wherein the operating element and prime mover (not shown) are mounted on the distal portion of a hollow extension member 210. A pair of conduits 263 and 264 extend through member 210 and communicate with passageways 234a and 235 as well as with appropriate ports of the prime mover. Passageway 234a is not continuous with return passageway 234, but communicates therewith only through bore 236. Spool 237 is provided with a land 267 to close passageway 234a, and this land includes a tapered shoulder at 268 to provide a throttling or gradual hydraulic braking action as spool 237 is moved to close off passage 234a and open passage 236a when the trigger 249 is released. This provides gradual braking for tools which develop high momentum when in use, for instance limb saws of the type having rotary cutting blades.

FIGS. 7 through 10 illustrate another modification of a handle control unit in accordance with this invention for use in a hydraulically powered drill or impact wrench having a reversible rotary drive motor 361. Here a hous ing 324 includes a handle or hand grip portion 332 provided with passageways 334 and 335, a bore 336 receiving a valve 337, and connectors 356 and 357 generally as in previously described embodiments. A finger rest, or trigger member 349 is mounted directly on spool 337 for convenient operation by the operator with fingers of the hand gripping handle 332. A subplate portion of housing 324 is formed with passageways 364a and 36412 communicating with two pressure or drive ports (not shown) of a reversible motor 361 and with a second valve bore 370 in housing 324, see FIG. 8. A cooperating rotatable spool 3'71 disposed in bore 370 includes an end groove at 372 and an axial bore at 373 in communication with pressure supply passageway 335. A bore 374 extends radially from bore 373 to selectively communicate with either passage 364a or 36412 as spool 371 is rotated. Hydraulic fluid under pressure may thus be directed to either pres sure port of the motor to drive motor 361 in either direction of rotation. Groove 375 communicates with passageway 334 and with a return passage 376 communicating with the exhaust port of motor 370 in both positions of valve spool 371. A lever 377 provides convenient means for rotating spool 371 and indicating its position.

It will be ovious that certain other modifications of the embodiments shown may be made by those skilled in the art, particularly in light of the foregoing disclosure, without departing from the spirit and scope of this invention.

It will thus be seen that improved power tools have been provided which may be designed to provide high power output, but which are compact and may be made relatively light in weight and convenient to handle as a hand tool. Tools in accordance with this invention are streamlined and have a minimum number of projections or other parts which might become entangled or caught on extraneous objects in the environments of use. At the same time these tools are designed of a minimum number of parts, of rugged construction, to provide long trouble-free use, and are simple and economical to produce. In one embodiment a tool is provided with a recip rocating drive piston which is positively driven in both directions of movement with a simple construction having a minimum number of parts and which is of rugged construction and economical to produce. These results are obtained with tools having open-center control valves thereby providing improved hydraulic hand tools which may conveniently be incorporated in a compact, portable hydraulic tool system.

While particular embodiments of this invention are shown above, it will be understood of course that the invention is not to be limited thereto since many modifications may be made by those skilled in this art, particularly in light of the foregoing teachings. It is contemplated therefore by the appended claims to cover any such modifications as fall with the true spirit and scope of this invention.

I claim:

1. A power tool comprising a tool head having a reciprocable operating mechanism, a hydraulic cylinder having end walls, a reciprocable piston in said cylinder, a connecting rod having one end connected to said piston, said rod extending through one of said end walls and being connected to said operating mechanism, a hydraulic fluid supply conduit communicating with the end of said cylinder corresponding to said one end wall, a hydraulic fluid return conduit communicating with the other end of said cylinder, valve means for selectively permitting or preventing passage of fluid between said conduits, whereby said piston may be moved toward said other end of said cylinder by hydraulic fluid supplied through said supply conduit when said valve is positioned to prevent passage of fluid between said conduits and a relief valve in said return conduit on the side of said valve means opposite said cylinder for maintaining a predeter mined minimum hydraulic pressure in said cylinder to move said piston toward said one of said end walls when said valve means is positioned to permit flow of fluid between said conduits.

2. A power tool as in claim 1, including a spring between said piston and said other end of said cylinder.

3. A power tool as in claim 2 wherein said spring is carried on said piston and is of a length to be compressed between said piston and said other end only during a portion of the piston reciprocating movement.

4. In a hydraulic tool having a reciprocable operating mechanism; an elongated integral housing defining a hydraulic cylinder at one end, a hand grip for the operator of said tool at the other end, and a pair of passageways communicating with opposite ends of said cylinder and extending through said hand grip to said other end; means at said other end of said housing for connecting external hydraulic fluid conduits in communication with said pair of passageways; a reciprocable piston in said cylinder; a connecting rod connected at one end to said piston, said rod extending through one end of said cylinder and being connected to said operating mechanism; a relief valve in the passageway of said pair of passageways communicating with the other end of said cylinder for maintaining a predetermined minimum hydraulic pressure in said cylinder; and valve means disposed between said relief valve and said cylinder for selectively permitting or preventing passage of fluid between said pair of passageways to move said piston toward said other end of said cylinder by hydraulic fluid supplied through said passageway communicating with said one end of said cylinder when said valve means is positioned to prevent passage of fluid between said pair of passageways and to move said piston towards said one end of said cylinder when said valve means is positioned to permit passage of fluid between said pair of passageways.

5. A hydraulic tool comprising an elongated housing having first and second ends and defining a hydraulic cylinder adjacent said first end, a hand grip adjacent said second end, a pair of passageways communicating with opposite ends of said cylinder and extending longitudinally of said housing to said second end, and a valve bore communicating with said pair of passageways, a reciprocable piston in said cylinder, a connecting rod having one end connected to said piston and extending through one end of said cylinder, a driven tool unit secured to said housing and having a reciprocable operating mechanism connected to said connecting rod, connecting means on said second end of said housing for connecting external hydraulic fluid conduits in communication with said passageways, means in said valve bore for selectively permitting or preventing passage of fluid through said bore between said passageways to move said piston toward the other end of said cylinder by hydraulic pressure supplied through said passageway communicating with said one end of said cylinder when said means prevents passage of fluid through said bore, and a relief valve in the passageway of said pair communicating with the other end of said cylinder, between said valve bore and said connecting means, for maintaining a predetermined minimum hydraulic pressure in both ends of said cylinder to move said piston toward said one end when said valve means is positioned to permit fluid flow through said bore.

6. A hydraulic drive system including a hydraulic cylinder, a reciprocable piston dividing said cylinder into first and second chambers, a connecting rod connected to said piston and extending through said first chamber and one wall thereof whereby the effective area of said piston is lesser on the side thereof corresponding to said first chamber than on the side corresponding to. said second chamber, a hydraulic fluid supply conduit communicating with said first chamber, a hydraulic fluid return conduit communicating with said second chamber, first means for selectively permitting or preventing flow of fluid from said supply conduit to said return conduit, whereby said piston may be moved toward said second chamber by hydraulic fluid supplied through said supply conduit when said first means prevents flow of fluid between said conduits, and means for maintaining a predetermined minimum hydraulic pressure in both of said chambers to move said piston toward said first chamber when said first means permits passage of fluid from said supply conduit to said return conduit.

7. A power tool adapted to be hand held by the operator when in use comprising a cylinder, a reciprocable piston in said cylinder, an elongated handle member secured at one end to said tool and protruding therefrom to form a hand grip for the operator of said tool, a connecting rod connected at one end to said piston and extending through one end wall of said cylinder, a supply conduit communicating with said one end of said cylinder and extending longitudinally through said handle member, a return conduit communicating with the other end of said cylinder and extending longitudinally through said handle member, a valve bore in said handle member communicating with said pressure and return conduits, control valve means in said bore for selectively preventing or permitting passage of fluid therethrough between said conduits, for supplying fluid under pressure to said one end of said cylinder to move said piston toward said other end of said cylinder when said control valve means is positioned to prevent flow through said bore, trigger means adjacent said handle member for selectively positioning said control valve means, connecting means at the distal end of said handle member for connecting external hydraulic fluid conduits with said supply and return conduits, and a relief valve in said return conduit between said valve bore and said connecting means for maintaining a minimum hydraulic pressure in both ends of said cylinder to move said piston toward said one end of said cylinder when said control valve means is positioned to permit flow of hydraulic fluid through said bore between said supply and return conduits.

References Cited UNITED STATES PATENTS 866,370 9/1907 Leineweber 173--170 1,419,269 6/1922 Keller 173--137 1,524,497 1/ 1925 Astrom et al.

2,075,341 3/1937 Goodman 173169 2,385,419 9/1945 Matulich 173-169 2,446,011 7/1948 Johnson et al. 173-169 2,709,446 5/1955 Miller 173--169 3,099,135 7/1963 Hoodley 173169 3,236,156 2/1966 Carnescea et al. 91-468 FRED C. MATTERN, JR., Primary Examiner.

L. P. KESSLER, Assistant Examiner. 

1. A POWER TOOL COMPRISING A TOOL HEAD HAVING A RECIPROCABLE OPERATING MECHANISM, A HYDRAULIC CYLINDER HAVING END WALLS, A RECIPROCABLE PISTON IN SAID CYLINDER, A CONNECTING ROD HAVING ONE END CONNECTED TO SAID PISTON, SAID ROD EXTENDING THROUGH ONE OF SAID END WALLS AND BEING CONNECTED TO SAID OPERATING MECHANISM, A HYDRAULIC FLUID SUPPLY CONDUIT COMMUNICATING WITH THE END OF SAID CYLINDER CORRESPONDING TO SAID ONE END WALL, A HYDRAULIC FLUID RETURN CONDUIT COMMUNICATING WITH THE OTHER END OF SAID CYLINDER, VALVE MEANS FOR SELECTIVELY PERMITTING OR PREVENTING PASSAGE OF FLUID BETWEEN SAID CONDUITS, WHEREBY SAID PISTON MAY BE MOVED TOWARD SAID OTHER END OF SAID CYLINDER BY HYDRAULIC FLUID SUPPLIED THROUGH SAID SUPPLY CONDUIT WHEN SAID VALVE IS POSITIONED TO PREVENT PASSAGE OF FLUID BETWEEN SAID CONDUITS AND A RELIEF VALVE IN SAID RETURN CONDUIT ON THE SIDE OF SAID VALVE MEANS OPPOSITE SAID CYLINDER FOR MAINTAINING A PREDETERMINED MINIMUM HYDRAULIC PRESSURE IN SAID CYLINDER TO MOVE SAID PISTON TOWARD SAID ONE OF SAID END WALLS WHEN SAID VALVE MEANS IS POSITIONED TO PERMIT FLOW TO FLUID BETWEEN SAID CONDUITS. 